Processing multiple files

Could anyone point me in the right direction as far as finding info on
processing multiple files?

Background: We have a multi-system application that includes an art
management system, content management system, and publisher (to PDF). A new
output format is being added, and I've been told that I need to develop a
stylesheet that can loop through a directory of files and process each in
turn, adding them to a single output file.

So, for example, I have DirectoryA, which contains PubFile1,
ActivityFile1A, ActivityFile1B, PubFile2, ActivityFile2A, ActivityFile2B.
What I need to do is take a few pieces of info from PubFile1 and start an
output file (Output1). Then, I need to loop through all related
ActivityFile documents and add the output of those to the Output1 file.
When that is complete, I need to move on to PubFile2 and create Output2,
looping through the ActivityFile2 docs and adding their output to Output2.

I'm very new to XSLT, and although I've gotten to be fairly comfortable
navigating through single input docs, I have no idea how to go about this.
Any help would be much appreciated (since I'm on the digest, an off-list
reply would be even more appreciated...)
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